1. Field of the Invention
The present invention relates to (1) a novel ester compound, (2) a polymer comprising units of the ester compound which is blended as a base resin to formulate a chemically amplified resist composition suitable in the micropatterning technology, (3) a resist composition comprising the polymer, and (4) a patterning process using the resist composition.
2. Prior Art
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers comprising cycloaliphatic compounds in the backbone are under investigation. In either case, the basic concept is that some or all of alkali soluble sites of alkali soluble resin are protected with acid labile or eliminatable groups. The overall performance of resist material is adjusted by a choice from among a variety of acid eliminatable protective groups.
Exemplary acid eliminatable protective groups include tert-butoxycarbonyl (JP-B 2-27660), tert-butyl (JP-A 62-115440 and J. Photopolym. Sci. Technol. 7 [3], 507 (1994)), 2-tetrahydropyranyl (JP-A 2-80515 and JP-A 5-88367), and 1-ethoxyethyl (JP-A 2-19847 and JP-A 4-215661). While it is desired to achieve a finer pattern rule, none of these acid eliminatable protective groups are deemed to exert satisfactory performance.
More particularly, tert-butoxycarbonyl and tert-butyl are extremely less reactive with acids so that a substantial quantity of energy radiation must be irradiated to generate a sufficient amount of acid in order to establish a difference in rate of dissolution before and after exposure. If a photoacid generator of the strong acid type is used, the exposure can be reduced to a relatively low level because reaction can proceed with a small amount of acid generated. However, in this event, the deactivation of the generated acid by air-borne basic substances has a relatively large influence, giving rise to such problems as a T-top pattern. On the other hand, 2-tetrahydropyranyl and 1-ethoxyethyl are so reactive with acids that with the acid generated by exposure, elimination reaction may randomly proceed without a need for heat treatment, with the result that substantial dimensional changes occur between exposure and heat treatment/development. Where these groups are used as protective groups for carboxylic acid, they have a low inhibiting effect to alkali dissolution, resulting in a high rate of dissolution in unexposed areas and film thinning during development. If highly substituted polymers are used to avoid such inconvenience, there results an extreme drop of heat resistance. These resins fail to provide a difference in rate of dissolution before and after exposure, resulting in resist materials having a very low resolution.
For the above-described resist materials using acid-eliminatable protective groups, there is a common problem of line density dependency that when a pattern to be transferred includes dense and sparse regions, it is impossible to produce the desired pattern in both the dense and sparse regions at the same exposure. More particularly, with respect to the formation of a line-and-space pattern, for example, if solitary lines are formed at an exposure that can resolve crowded lines with good size control, they are finished to a line width less than the desired size. Presumably this phenomenon is ascribable to the increased diffusion length of acid generated upon exposure. There is a tendency that the diffusion of the generated acid is enhanced as the system becomes more hydrophobic. Since both (meth)acrylic resins and cycloaliphatic backbone resins have increased their carbon density in order to improve etching resistance, the diffusion of the generated acid is more promoted as a result, exaggerating line density dependency. Then at the very fine pattern size for which an ArF excimer laser is actually used, a resist material having substantial line density dependency cannot be used in an industrially acceptable manner because solitary lines can disappear. While a finer pattern rule is being demanded, there is a need to have a resist material which is not only satisfactory in sensitivity, resolution, and etching resistance, but also minimized in line density dependency.